Although the present invention has application in the prevention of discharges and accidental mixing of many liquids, for simplicity, its application in the prevention of oil discharges will be described.
As is generally known, most oils are immiscible with water and, due to the lower density of the former, tend to rise to the surface of the water.
When storing large quantities of oil, it is often the case that small quantities of water condense or otherwise settle out of the oil, thereby forming a water table at the bottom of the container in which the liquids are stored. It is often desirable to remove such water tables. In order to draw water from the bottom of oil storage tanks, water draw-off pipelines are often located at or near the bottom of such tanks. Such pipelines are provided with either manual or automatic closing valves by which means discharge may be stopped when oil becomes present in the line.
Many such automatic closing valves are known. The most common types of these valves generally comprise a vessel having an inlet, an outlet and a specific gravity sensitive float that is less dense than water but more dense than oil. Typically in such valves, the float floats freely within the vessel when the vessel is filled with water. The float sinks when the vessel is substantially filled with oil, thereby closing the outlet and stopping further liquid discharge from the device.
A problem of such devices is that the height at which the outlet closing float floats within the vessel is proportional to the ratio of oil to water within the vessel. Therefore, in such devices if oil slowly enters into the vessel the float will slowly sink, and accordingly will slowly shut off discharge.
Another problem of such devices is that as the oil/water level slowly lowers within the vessel the float will reach a neutral buoyancy within the liquid. As such, the weight (downward force) of the float becomes nearly zero. If any obstruction (twig, hair, etc.) should become located underneath such a float, the downward force may not be sufficient to overcome the obstruction and the device would malfunction.
Similarly, if even a small object were to come to rest on top of such a float, it may be sufficient to sink it, prematurely closing the discharge outlet.
Another problem of such devices is that when the oil/water level in the vessel is at a height which positions the float within close proximity to the discharge outlet, high velocity flow between the float and discharge may effect a venturi flow situation and thereby cause the float to sink prematurely and at high speed.
Another problem of such devices is that a large quantity of oil must be present inside the vessel in order to cause the float to close the discharge outlet.
Another problem of such devices in which flow is controlled by closing the outlet is that inlet and outlet fluid pressures and flow rates must be closely regulated so that flow rate out of the device not be greater than flow rate into the device. When such is not the case, the water level in the device lowers, which causes the float to lower and close off the device--even if there is no oil present. For the same reason, cutting off the flow of liquid into such devices will cause the device to close. In most cases, valves of this type once closed will not reopen by simply refilling the vessel with water.
It is a further problem of such devices that in the presence of oil, only flow out of the device is stopped and flow from storage tank may only be controlled if said vessel is a "closed" container.
It is a further problem of such devices that they must be closely examined to determine whether or not valve has actuated (closed) and whether or not oil is present within the device.
It is a further problem of such devices that, in many cases, a long multiple sequence operation is required in order to "reset" the devices once they have been closed.
It is a further problem of such devices in which only flow from the device is controlled, that in the event of equipment failure (for example, a leak in the vessel), flow from the storage tank will not be stopped.